1. Field of the Invention
The present invention relates to a method and apparatus for contacting gas and liquid. More particularly, but not exclusively, it relates to a method of and apparatus for initiating and at least partially effecting a fermentation or bioreaction process in which a gas such as oxygen or an oxygen containing gas, is introduced into a liquid, which may contain a solid phase therein. The reactor into which the gas and liquid are fed may contain a micro-organism. aerobic in the case of the gas being oxygen, which causes a reaction of the solid or liquid phase.
As stated above, the gas used is preferably oxygen or an oxygen containing gas, such as air. However, the invention is not limited to use of air or oxygen, and other gases may be used within the scope of the present invention. Similarly, the liquids involved are usually aqueous, but other liquids, which term includes mixtures of liquids or emulsions or dispersions, also fall within the scope of the present invention. In this connection, the term liquid will be used herein for convenience to refer to such liquids and to liquids containing solids in the form of suspensions or solutions.
2. Discussion of the Background
It is known to carry out such bioreactions in a stirred vessel, wherein air is introduced by means of sparge pipes into a tank containing liquid (as defined above as including possibly solid phases). After a period of time, the chosen micro-organism will have effected sufficient reaction for clear or treated liquid to be run off, possibly after a settling stage. One disadvantage with this process is the length of time taken to effect the reaction, which may be as long as 24 hours, and of course the amount of space needed to accommodate the volume of liquid/solids to be treated. It would be advantageous if a similar reaction could be accomplished within a shorter time and also using an apparatus which occupies a smaller volume. Indeed, it would also be advantageous to effect an improved gas/liquid contact in a short time, possibly as little as 20 seconds, and then allow a bioreaction to take place at a slower rate in a second or subsequent reactor. The improved gas/liquid contact and possible recirculation may enable overall treatment times to be reduced considerably, say from 24 hours to 2 hours.
So-called waterfall columns are known in which a jet of liquid is introduced into a gas filled space, entraining some of the gas to form a shallow froth above the liquid in the reactor.
It is known from British Patent No 1596738 to mix gas and liquids more effectively by introducing them co-currently in a downward flowing reactor. Such mixing however, can still create a gas space above the froth, although it can produce gas bubbles of such a size that the interfacial area between gas and liquid is increased, and therefore the contacting process is improved. Use of such apparatus, however, produces a zone of froth at an uppermost part of the reactor column wherein gas/liquid contact and mass transfer is not at its most effective.
It is also known from U.S. Pat. No. 4,834,343 to increase further the contact between gas and liquid by introducing at least part of the flow in a direction tangential to the longitudinal axis of the reactor. However this process has been carried out only in reactor columns having a "swirl" zone of substantially constant diameter and volume. This "swirl" zone is formed by the mixture in that zone of a vertically downward fluid stream at the top of the column, with a tangential inlet below it. This has been found to be unsatisfactory.